The present invention relates to electronic circuits, and more particularly to minimization of DC component of a feedback signal in an amplifier.
FIG. 1 shows a transimpedance amplifier 10, as known in the prior art. Resistor 16 represents the resistance of a sensor, such as a reader head (not shown), and resistor 14—shown using the dotted line—represents the output impedance of amplifier 10. Transimpedance amplifier 10 is adapted to amplify and convert the current signal generated by resistor 16 into a voltage signal VOUT. The DC operating point of the reader head is ideally established by the biasing current source 18 only. However, the DC component of the feedback current signal IFB also contributes to and thus disturbs the DC operating point of the reader head via node N1.
The input impedance Rin as seen from input node N1 is approximately defined by the following expression:
                              R                      i            ⁢                                                  ⁢            n                          =                              1                          G              m                                +                      R                          1              +                              Gm                ×                                  R                  14                                                                                        (        1        )            where Gm is the voltage-to-current gain of transimpedance amplifier 10. Similarly, the output impedance Rout as seen from input node N2 is approximately defined by the following expression:
                              R          out                =                  1                      G            m                                              (        2        )            
The closed-loop voltage-to-current gain
      V    out        I          i      ⁢                          ⁢      n      of amplifier 10 is defined by the following expression:
                                          V            out                                I                          i              ⁢                                                          ⁢              n                                      =                  -                      R            12                                              (        3        )            
One conventional technique for reducing the above-described disturbance of the DC operating point of the reader head is to sense or estimate the input voltage Vin and use the sensed or estimated voltage to reduce the flow of the feedback current IFB to node N1. To sense the input voltage Vin, a control loop with a negative feedback is used. Such control loops are often complex and need to be carefully designed to remain stable during operation. Furthermore, amplifier 10 may need to be modified to accommodate such control loops. Currently known estimating techniques for estimating voltage Vin also suffer from inaccuracies and thus may not reduce the flow of feedback current IFB to node N1 under all required operating conditions. A need continues to exist for a feedback circuit disposed across an amplifier that does not disturb the DC operating point of the input circuitry of the amplifier and that does not degrade the small-signal operation of the amplifier.